1. Field of the Invention
The present invention relates generally to a finder optical system for single-lens reflex cameras, and more particularly to a finder optical system wherein a part of an eyepiece is moved in the optical axis direction of a finder for diopter movement.
2. Prior Art
In the case of finders of optical devices, binoculars, microscopes, etc., images formed by optical systems at predetermined positions are generally viewed by the eye. If the positions of the images are located outside the range of ocular accommodation ability, observation then becomes difficult. Required to observe these images is a diopter movement means for move them to a position where they are easily observable depending on ocular accommodation ability.
According to one typical diopter movement means known for the finder of a single-lens reflex camera, the eyepiece is moved back and forth with respect to the finder image.
In the finder of a conventional 35 mm single-lens reflex camera, however, the focal length f.sub.e of the eyepiece must be f.sub.e .gtoreq.about 60 mm, because of the presence of a pentaprism between the finder image and the eyepiece. This leads to a serious problem or an increase in the quantity of movement of the eyepiece required for obtaining a desired diopter change. That is, the quantity .DELTA. of movement of an eyepiece per diopter (m.sup.-1) is given by EQU .DELTA.=f.sub.e.sup.2 /1000
Here, if it is intended to obtain a diopter change of at least 2 diopters (m.sup.-1), then the required quantity of movement of the eyepiece is about 7 mm. However, a 35 mm single-lens reflex camera cannot have a space wide-enough for such large movement, because the eyepiece must be located in a very limited space in the rear of the pentaprism.
One approach to reducing the quantity of movement is typically disclosed in JP-A 54(1979)-126530 which is directed to a diopter movement system comprising two lens units, i.e., a positive lens unit and a negative lens unit in order from the eye point side, wherein diopter movement is achieved by moving only one of these lenses while the other remains fixed. Since the focal length of the movable lens is shorter than the overall focal length of the eyepiece, some considerable diopter change is achievable by a slight movement.
For similar purposes and in order to make satisfactory correction for various aberrations, an eyepiece system comprising, in order from the eye point side, a first positive lens unit, a second positive lens unit and a third negative lens unit with the second positive lens unit taking part in diopter movement, is proposed in JP-B 60(1985)-48730. Another eyepiece system comprising, in order from the eye point side, a first positive lens unit, a second positive lens unit and a third negative lens unit with the third negative lens unit adapted for diopter movement is disclosed in JP-B 61(1986)-19969.
However, the diopter movement system disclosed in JP-A 54(1979)-126530 has the following problems. When one of the two positive and negative lenses is moved on the finder optical axis with the other remaining fixed for diopter movement, aberrations, especially off-axis aberrations such as distortion and coma cannot sufficiently be corrected in the reference state (0 m.sup.-1). Diopter movement, if carried out, makes aberration variations likely to become large in association with lens movement. Such aberration variations occur for the following reason.
In the case of a lens system of simple construction like an eyepiece, its overall aberrations are generally eliminated or reduced by allowing aberrations resulting from the positive lens to be offset by the negative lens. However, it is only when both the lenses are located at proper relative positions that the aberrations produced by the two lenses can be offset each other; that is, when the lenses are located at improper positions, portions of the aberrations that cannot be offset each other manifest themselves as the overall aberrations of the eyepiece. In other words, there are aberration variations incidental to the lens movement for diopter movement. Especially when it is intended to increase the eye relief (the distance from the eye point to the surface of the eyepiece proximate to the eye point side) or enhance the rate of field of view, much difficulty is involved in make sufficient correction for off-axis aberrations such as distortion and coma, because rays, when passing through the lenses, are away from the optical axis.
With the system of JP-B 60(1985)-48730 which comprises the first lens in the form of a positive lens, the second lens in the form of a positive lens and the third lens in the form of a negative lens with the second lens adapted for diopter movement or with the system of JP-B 61(1986)-19969 which comprises the first lens in the form of a positive lens, the second lens in the form of a positive lens and the third lens in the form of a negative lens with the third lens used for diopter movement, it is somehow possible to make satisfactory correction for aberrations such as distortion and coma. To make more reliable correction for aberrations, however, it is impossible to reduce the focal lengths of the lenses extremely; that is, the quantity of the lens movement for diopter movement is increased. When a wide range of diopter movement is taken in account, there must be a space allowing for such movement, resulting in an increase in the overall length of the eyepiece and so making it impossible to achieve compactness.
Still another diopter movement system is disclosed in JP-A 5(1993)-181071, which comprises a first lens in the form of a positive lens, a second lens in the form of a positive lens and a third lens in the form of a negative lens and in which the first and second lenses are moved together in the optical axis direction for diopter movement. Now, let F denote the composite focal length of the first, second and third lenses and X stand for the distance of the third lens from its image-side surface to the image surface. Then, the value of X/F is as short as about 0.25 to about 0.4. In other words, this system is well fit for an electronic view finder, but is not suitable for an optical system such as a single-lens reflex camera in which an image inverter must be located between the image surface and the eyepiece.